Documents of value, such as bank notes or checks, often have magnetic security features. The magnetic security features can be hard features formed from magnetically hard substances having a high level of magnetic remanence and a high coercive field strength. The magnetic security features can also be soft features formed from magnetically soft substances having a high level of remanence but a low coercive field strength.
Magnetically soft substances are easily magnetized in a magnetic field. The magnetization of a magnetically soft substance can, for example, be produced by an electric current in a coil or by the presence of a permanent magnet. Magnetization in this manner produces a magnetic flux density in the magnetically soft substance which is many times higher than a magnetic flux density produced in air by the same magnetic field. Magnetically soft substances generally have a coercive field strength less than 1000 A/m. Magnetically soft substances can, however, have saturation magnetization values which are comparable with magnetically hard substances so that, in the saturated state, they cannot be distinguished from magnetically hard materials.
Magnetically hard materials are magnetized before use and a magnetic field is derived from magnetically hard security features without the presence of an external magnetic field. This magnetic field remains in existence for a relatively long time. However, over time, the magnetically hard materials are susceptible of becoming demagnetized. For example, during handling, bank notes are often subjected to impacts or are creased which may result in a demagnetization. It is, therefore, known to use a pre-magnetizing magnet to allow the magnetically hard security feature to retain magnetization over a longer time period.
Ferromagnetic security threads used on bank notes are disclosed in DE 1696245. Ferromagnetic materials used to form the security thread can have either a small coercive field strength or a large coercive field strength. Measuring devices for testing such bank notes identify both security threads made of a material having a small coercive field strength and security threads made of a material having a large coercive field strength.
A measuring device for measuring magnetic properties is disclosed in WO 2010/006801. The measuring device has a sensor cell comprising at least two magnetoresistive sensors which are arranged along a line direction and a support field device which produces a magnetic support field. The support field device has a magnetic field component pointing in the line direction and a field strength varying in the line direction. The measuring device has a pre-magnetizing magnet disposed upstream with respect to the sensor line. The pre-magnetizing magnet first magnetizes a measurement object with a magnetic pattern and the measurement device is subsequently guided over the sensor line to identify the magnetic pattern of the measurement object.
In an embodiment of the measuring device disclosed in WO 2010/006801, two sensor lines may be arranged one after the other in a reading direction of a plurality of measurement objects. The sensor lines are used to detect different magnetic patterns. However, the sensor line which identifies a magnetically hard pattern must not have a pre-magnetizing field during the measurement, while in order to identify the magnetically soft patterns, a powerful pre-magnetizing field for the sensor line identifying a magnetically soft pattern is necessary. The pre-magnetizing field acts in the region of a first sensor line and magnetizes hard and soft magnetic features, allowing the identification of the magnetically soft and hard patterns. If the bank note is subsequently guided over a second sensor line, in the region of which the pre-magnetizing field no longer acts, this second sensor line can separately identify the purely magnetically hard pattern.
Another measuring device for measuring magnetic properties is disclosed in EP 0977015. The measuring device has a measuring element, a support field magnet, and a pre-magnetizing magnet. Magnetic material on a document of value has to be magnetized before it is identified.
In the prior art, documents of value have a plurality of magnetically hard security features in which one magnetically hard security feature comprises a first magnetic material having a first coercive field strength and a second security feature comprises a second magnetic material having a second coercive field strength. Some known documents of value have combined security features produced by arranging a material with a high coercive field strength adjacent a material with a low coercive field strength.
The presence of the combined security features can be established in such documents of value with a plurality of measuring devices each having a plurality of sensor lines and a plurality of pre-magnetizing magnets in a travel direction of the document of value. All the security features are pre-magnetized in the same direction with a first pre-magnetizing magnet of a first measuring device. As a result, it is possible to initially establish with a first sensor line of the first measuring device whether the document of value has the requisite number of security features. If the document of value is then pre-magnetized with a second pre-magnetizing magnet of a second measuring device, which produces a weaker field than the first pre-magnetizing magnet so that the security feature with the weakly coercive material is magnetized in the opposite direction, it can be determined in a subsequent sensor line which security features are highly coercive and where they are located on the document of value. The security features made of highly coercive material are still pre-magnetized by the first pre-magnetization in the first direction and can be identified by the sensor line. The security features with a weakly coercive material or the combined security features are re-magnetized or cancel each other out. These security features cannot be identified directly by the second sensor line.
As a result of the comparison of the measurement result of the first sensor line and the measurement result of the second sensor line, it can be determined which security features on the document of value are made of a highly coercive material and which are made of a weakly coercive material. The measuring device is required to be very long in the travel direction of the document of value. In the majority of applications, in particular in cash dispensers, however, there is generally little space for a measuring device having a pre-magnetizing magnet and a double-line sensor line.